Protective coating for ferrous metal conduit



May 31, 1966 F. J. KENNEDY 3,254,005

PROTECTIVE COATING FOR FERROUS METAL CONDUIT Filed Jan. 25, 1962 2 Sheets-Sheet 1 INVENTOR. 82 X \\&W\ y m xmxwm wzEE: 342%"; a mw i a llll I Q\ A7 TORNEYS 2 Sheets-Sheet 2 F. J. KENNEDY 2: 3 v wnw PROTECTIVE COATING FOR FERROUS METAL CONDUIT u ma May 31, 1966 Filed Jan. 25, 1962 INVENTOR.

BY AT TORNEYS United States Patent 3,254,005 PRQTECTIVE COATING FOR FERROUS METAL CONDUIT Frank J. Kennedy, Mount Lebanon, Pittsburgh, Pa., as-

signor to H. K. Porter Eompany, Inc., Pittsburgh, Pa.,

a corporation of Delaware Filed Jan. 25, 1962, Ser. No. 168,658 12 Claims. (Cl. 20437) This invention relates to the protection of ferrous metal bodies from corrosion, and more especially to the application of protective coatings to :ferrous conduits, such as are used for electrical conductors. Such conduits are bent to short radius curves when'installed in buildings and it is important to have a protective layer that does not crack or spall off when the conduit is bent sharply.

It is an object of this invention to provide an improved process for applying a protecting layer of Zinc to the surface of a ferrous metal conduit or other work piece, and to alloy the Zinc layer to the ferrous metal so as to obtain a maximum adherence. While Zinc is the preferred coating it will be understood that the term Zinc includes the chemical equivalents of zinc.

Another object is to obtain a method for alloying zinc coating that is more economical to apply as compared with previously known comparable coatings. Still another object is to obtain a nonporous coating from an acid plating bath in place of previous coatings that require the application of two layers of zinc, one from an alkaline plating bath and the other from an acid plating bath.

By applying all of the Zinc from an acid electrolyte, this invention dispenses with the use of cyanide in the elec trolyte and thereby eliminates the necessity for an expensive waste treatment system. It also eliminates Zinc anodes in ball form essential to the cyanide electrolyte and substitutes slab zinc in the acid electrolyte thereby effecting important economies in the raw material cost and the subsequent labor cost required for organizing ball anodes in the cyanide solution. The invention dispenses with the high cost of cyanide solution for the less expensive acid electrolyte.

A further object is to provide a conduit surface in the heat treated condition that insures a tight bonding for paint or enamels as increased protection against corrosive influences. The method of applying the zinc and establishing the Zinc iron alloy accomplishes this objective.

A further objective is to eliminate the disadvantages of retort sherardizing which included wasteful practices in zinc dust and free iron in the coating. The unequal distribution of the coating resulted in frequent spalling of the zinc. All of those disadvantages are eliminated by this invention which includes major manpower savings. As compared to retort sherardizing this invention reduces processing time. The sherardizing process consumed about 3 /2 days. The new process consumes one day.

Another object is to obtain more uniform alloying of the coating of zinc over the surface of the work piece by controlling the reflective qualities of the surface and the resulting rate of heating by radiant heat.

Still another object is to obtain aplated ferrous metal body, and especially a section of electrical conduit, with a nonporous coating that is a single layer of acid deposited zinc alloyed to the ferrous metal substantially uniformly over the entire area of the outside surface of the section of conduit.

Other objects, features and advantages of the invention will appear or be pointed out as the description proceeds.

In the drawing, forming a part hereof, in which like reference characters indicate corresponding parts in all the views; FIGURE 1 is a flow diagram showing the successive steps of the process of this invention;

FIGURE 2 is a diagrammatic view showing apparatus for performing the successive steps shown in the flow diagram of FIGURE 1;

FIGURE 3 is a top view, partly broken away and in sect-ion, showing a portion of the apparatus illustrated in FIGURE 2;

FIGURE 4 is an elevation, mostly in section, of the apparatus shown in FIGURE 3;

FIGURE 5 is a sectional view taken on the line 55 of FIGURE 4;

FIGURE 6 is a sectional view taken on the line 6-6 of FIGURE 3;

FIGURE 7 is an enlarged detail view showing a portion of the conveyor chain illustrated in FIGURES 3-6;

FIGURE 8 is a perspective view of one of the sections of conduit which is made by the process illustrated in the other figures; and

FIGURE 9 is a greatly enlarged sectional view through a portion of the wall of the conduit shown in FIGURE 8.

Zinc can be deposited very rap-idly from an acid electrolyte, but one of the problems presented has been that the zinc coating from an acid electrolyte did not have the fine-grained and small-crystalline structure necessary for a good protective coating. This invention obtains a protective coating which is fine-grained and of small-crystalline structure, even though deposited from an acid electrolyte, by using a particular acid electrolyte.

This new result is obtained by using a formulation within the range specified below 3540 oz./ gal. ZnSO -7H O 3-4 oz./ gal. NH CI 12 OZ./ga1. A12(SO4)3 .2-.5 oz./ gal. NaF pH=3.5-4.5 free acid H This zinc sulphate electrolyte permits the zinc to be deposited with current densities ranging from'25 amperes to 200 amperes per square foot. The plating is continued until the zinc coating reaches a minimum thickness of at least 0.0002 inch; the time depending upon the current density.

For practical purposes the plating should continue only to a Zinc coating thickness of approximately 0.00l inch. While thicker coatings can be applied, they are unnecessary for the purposes of this invention and defeat the economy of the process by using more zinc than necessary. The cleaning of the ferrous metal conduits and the details of the plating process, other than the formulation of the bath, are in accordance with conventional practice and no further detailed description of them are necessary for a complete understanding of this invention.

FIGURE 2 shows the plating apparatus diagrammatically. Sections of conduit 12 are moved to a plating vat 14 by a conveyor 16. This plating vat and the conveyor may be constructed in accordance with the dis closure of Patent No. 2,742,017. The speed of operation of the conveyor 16 is correlated with the length of the vat 14 and the current density within the electrolyte so as to obtain the desired thickness of coating by the time the conveyor removes the conduits 12 from the electrolyte, designated by the reference character 18.

The plated conduits are conveyed past a station at which they are washed by spray pipes 20; and the conveyor 16 then moves the sections of'conduit 12 into another vat 24 containing a solution 26 which blackens or darkens the surface of the plated conduits. This blackening or darkening of the surfaces of the conduits is for the purpose of making them absorb heat at a uniform rate when subjected to a subsequent heat treatment using radiant heat. The conduits as they come from the plating vat 14 are ordinarily not of uniform reflective quality throughout their areas and when they are suband the size and wall thickness of the conduits.

jected to radiant heating, they heat with substantial variations in temperature between the different portions which are of dark or light color.

Various chemical baths can be used in the vat 24 for the reflectivity treatment. In the preferred embodiment of the invention, the material 26 in the vat 24 consists of 25 g./l. cupric salt in an acidified solution such as one-half per cent nitric acid. This reflectivity treatment not only makes the conduits have a uniform absorption surface but it also accelerates the heat absorption from radiant sources by providing a dark surface.

Other solutions can be used in the vat 24. For example, an acidic solution containing zinc and iron phosphates and also free phosphoric acid could be used. The conduits need not be dipped in the vat 24 and in place of this vat the conduits can be sprayed with the solution for darkening thesurface; but the vat 24 has the advantage 2 of obtaining uniform results without unnecessary waste of the treating solution.

The length of time that the conduit sections 12 are exposed to the action of the darkening solution in the vat 24 or the solution which is sprayed on the surface, must be long enough to produce the desired darkening and experience has shown that at least five seconds is necessary and that the use of a period of 30 to 45 seconds is satisfactory. Times longer than about one minute are objectionable, with the formulations described, because of waste of zinc.

Beyond the vvat 24, the conduit sections 12 pass another .spray pipe 20 which can be used for washing the surfaces of the conduits and beyond this washing station the conduit sections 12 travel along a conveyor 28 which deposits them on chain conveyors 30 that pass through furnaces 32 and 34 where the conduit sections 12 are subjected to heat treatment. These furnaces and the conveyor will be described in more detail in connection with other figures of the drawing. For the present, it is sufficient to understand that the conduit sections 12 are heated rapidly in the furnace 32 to a temperature at which alloying will take place, and the'conduit sections then pass through the furnace 34 where they are held at the desired temperature for sufficient time to permit the alloying to proceed.

In the furnace 32, the conduit sections 12 are subjected to radiant heat and they are heated rapidly to a temperature of at least 800 F. During this heating the outside surface of the conduits is heated to a higher temperature than the inside portions of the metal since a temperature gradiant is necessary to obtain the heat flow into the metal. Sometimes the outside surface may reach a temperature as high as 1000 to 1100 F., but only for a few seconds as this temperature would vaporize the zinc. The total heating time in the furnace 32 will vary from about forty seconds to two minutes depending upon the amount of heat generated by the radiant heat sources For example, a wall thickness of .109 inch is used for /2" conduit, whereas 5" conduits have a wall thickness of .258 inch.

The rate of heating and the time during which the conduit sections are exposed to the heat in the furnace 32 is correlated to bring the temperature of the sections to at least 800 and not more than 950 by the time the sections are removed from the furnace and the heat has had an opportunity to equalize throughout the wall thickness.

The temperature of the furnace 34 is maintained at least as high as 800 and not higher than 950. The length of time that the conduits remain in the furnace34 is between two and six minutes. The shorter time being used where the temperature is in the vicinity of 950 and the longer time being used where the temperature is nearer the lower end of the range.

' Since the same conveyor'30 moves the conduit sections 12 through the furnaces 32 and 34, it is necessary to have the length of each furnace correlated with the length of the other furnace and with the temperature of the other furnace so that the passage of a conduit section on the conveyor 30 will correspond with the proper heating time. Ordinarily this will require that the furnace 34 be longer than the furnace 32. Where greater flexibility is desired and the additional cost is not objectionable, the conyeyor 32 can be made in two sections, one of which moves the conduit sections through the furnace 32 and the other of which moves them through the furnace 34; and these two sections can run at different speed with appropriate facility for transferring each conduit from the first section of the conveyor to the second where the conveyors come together or overlap at the space between the furnaces 32 and 34.

Another modification which can be used is to have the furnaces 32 and 34 as a single unit with different zones of the furnace separated from one another by appropriate partitions and with the conduits passing from one 'zone to the next during their travel through the heating apparatus. The furnace 34 may be termed an oven.

After its travel through the furnace 34 or holding zone where the alloying actually occurs, each section of conduit is delivered by the conveyor 30 to a delivery table 40 from which the conduits may be taken to an inspection station or other facility for advancing them to the storage or shipping department.

The heating time and temperature ranges referred to above are sufficient to permit alloying to proceed from the zinc-ferrous metal interface through the entire thickness of the zinc coating, when the zinc coating is kept to a thickness not substantially greater than 0.001 inch. The alloying is not necessarily uniform, however, and the iron content of the alloy is sometimes higher near the zinc iron interface than toward the outside surface of the conduit. This is not objectionable.

FIGURES 3, 4 and 5 show the internal construction of the furnace 32. This furnace has a bottom 46, end walls 47 and 48, side walls 50 and a top 52. There are radiant burners 54 located in the bottom 46 and in the top 52. These radiant burners 54 are preferably of the type in which oxygen and fuel gas are burned inside the burner and the hot products of combustion are discharged across the face of the burner which is made of ceramic material and which is highly heated to a glowing temperature that radiates heat at a rapid rate to any object within the furnace 32. There are other and similar burners 54' in the walls of the furnace 32.

The conduit sections 12 enter the furnace 32 through an opening in the front wall 47 and they pass out of the furnace to an opening 62 in the back wall 48. The

I passes around a sprocket 66 supported by a shaft 68 to which power is applied to operate the conveyor 30.

The conveyor chain will be described in more detail in connection with FIGURE 7. The furnace 34 has a bottom 70, a front end wall 72, a back end wall 74, side walls 76 and a top 78. There is an entrance opening 80 similar to the opening 60 of the furnace 32 and there is an exit'opening 82 similar to the exit opening 62 of the other furnace. Within the furnace 34 there are partitions and ducts for the circulation of the hot air so as to maintain substantially uniform temperatures throughout the furnace 34 in spite of the openings 80 and 82 which must always be open for the entrance of successive conduit sections at one end of the furnace and for the discharge of the successive sections from the other end of the furnace.

The furnace 34 has three heaters 85, 86 and 87; and there is a centrifugal blower associated with each of these heaters. The blower of the heater discharges hot air through a duct 90 into the upper portion of the furnace 5. 34, and this duct 90 has an outlet 92 across its lower end near the top of the opening 80 in the front wall of the furnace. There is a bafile 94 for diverting the hot air from the duct 90 away from the opening 80 in the furnace wall.

Some distance back from the front wall of the furnace there is a duct 96 with an opening 98 in its lower wall for withdrawing air from the furnace and circulating it back to the heater 85. Both the duct 90 and the duct 96 are of progressively decreasing cross section toward the far wall which is most remote from the heater 85. There is space 99 between the ducts 90 and 96 throughout most of their length and this space 99 is partitioned off from the heating zone of the furnace by a partition 100.

The heater 86 and its associated blower force heated air into the furnace 34 from the opposite side of the furnace from the heater 85 and its associated blower. The ducts for the heater 86 and its blower include a duct 102 having an opening 104 through which heated air is discharged downwardly into the heating zone of the furnace; and include also another duct 106 having an opening 108 through which heated air is withdrawn from the heating zone of the furnace and through which it is sucked back to the blower of the heater 86. There are other bafiies 94 located in the openings 98, 104 and 108, and at other locations within the furnace 34 to deflect and control the circulation of the heated air.

The ducts 102 and 106 are of tapered cross sections and are spaced from one another throughout most of their length, the space being indicated by the reference character 110; and the space being partitioned off from the heating zone of the furnace by a partition 112.

The third heater 87 and its associated blower are located on the same side of the furnace as the heater 85 and they have similar ducts for supplying heated air to the furnace and for withdrawing air from the furnace for further heating by the heater 87. The ducts and openings for the heater 87 are indicated by the same reference characters as those of the heater 85 but with a prime appended.

After passing through the discharge opening 82 in the end wall 84, the conduit sections 12 are discharged onto the delivery table 40 as the chain 64 of the conveyor 30 starts around a sprocket 116 preparatory to its return to the front end sprocket 66.

There are two chains 64 of the conveyor 30. These chains are located a substantial distance apart, as is best shown in FIGURE and the conduit sections 12 are substantially longer than the space between the conveyor chains 64.

FIGURE 7 shows the construction of the conveyor chain 64. Each of the conveyor chains is made up of links 124 which have frame portions 126 extending upwardly to top edges 128 in which there are depressions 130 for holding the conduit sections. The upwardly extending portion 126 is an integral part of each of the links 124 and the links 124 are connected to their adjacent links 124 by other links 132. Each of the links 124 has wheels 136, there being one wheel on each side of the link for supporting the conveyor chains and the conduit sections load that it carries, from the furnace bottom, or tracks provided along the furnace bottom.

There are axles 138 for each of the wheels 136, these axles being stud shafts extending outwardly from the links 124. There are also pivot pins 140 located at opposite ends of the links 124 for connecting each of the links with the adjacent links 132 of the conveyor chain 64.

FIGURE 8 shows one of the conduit sections 12; and FIGURE 9 shows a sectional view through a wall of the conduit section after it has been heat treated. The ferrous metal is indicated in FIGURE 9 by the reference character 151 and the zinc coating is indicated by the reference character 152. The stippling through the zinc coating 152 indicates the alloying of the base metal 151 with the coating metal 152. The interface between the metal 151 and the metal 152 is indicated by a dotted line because there is an actual merging of the zinc into the ferrous metal, after the heat treatment, rather than a definite interface where the metal changes from zinc to iron. Because of this merging of the binc alloy 152 into the base metal 151, the conduit can be bent around curves of short radius and can be flattened to elliptical cross sections where necessary without a breaking of the coating metal 152 away from the base metal 151.

The preferred construction has been illustrated and described, but changes and modifications can be made and some features can be used in different combinations without'departing from the invention as defined in the claims.

What is claimed is:

1. The method of applying a uniform protective layer of zinc to the surface of a ferrous metal body, which method comprises electroplating a layer of zinc on the surface to be protected and with the ferrous metal body in an acid electrolyte, uniformly darkening the surface of said zinc by chemically treating it to bring the bare surface of said zinc to a state of substantially uniform reflectivity, heating the metal body and its protective layer by radiant heat and at a rapid rate in an enclosing space at a temperature substantially higher than the melting point of said zinc, transferring the ferrous metal body from the highly heated space to another space of lower temperature but at an alloying temperature of the ferrous metal and the zinc, and holding the ferrous metal body in the other space long enough to alloy said zinc and the ferrous metal for a substantial part of the thickness of the protective layer 2. The method of applying a protective layer of metal to a ferrous metal body as described in claim 1, and in which the protective layer that is applied to the ferrous body is zinc and the temperature of said other space is maintained between about 800 and 950 F.

3. The method described in claim 2 and in which the ferrous body is advanced through the higher temperature space and the other space at an equal and uniform speed, the temperature of the higher temperature space being proportioned to that of the other space to alloy the zinc through most of the radial thickness of the zinc by the time the ferrous metal body leaves said other space after entering at substantially the temperature to which it was heated in said higher temperature space.

4. The method described in claim 2, and in which the protective layer is zinc and the surface of the zinc is darkened by applying to it a cupric salt in an acidified solution.

5. The method described in claim 1, and in which the protective metal is zinc and higher temperature heating is done by radiant heat capable of heating the metal body to approximately 1100 F. but the ferrous metal body is withdrawn from the higher temperature heating space before the heating produces oxidation or vaporization of a substantial thickness of the zinc, and when the ferrous metal under the layer of zinc reaches a temperature of about 800 F., and the body is held in the other space and heated by convection currents not in excess of approximately 950 F.

6. The method described in claim 2, and in which the ferrous metal body is an electric conduit and it is heated by radiant heat in the higher temperature space to a temperature of at least 800 F. in a period of from forty seconds to two minutes, and it is then heated in the other space oven by holding at a substantially uniform range for a period of from two to six minutes.

7. The method described in claim 6, and in which the alloying is extended through at least most of the thickness of the zinc layer.

8. The method described in claim 7, and in which at least some degree of alloying is extended through the full thickness of the zinc layer.

9. The method described in claim 2, and in which the zinc is electroplated to a thickness of at least .0002 inch.

10. The method of applying a protective layer of zinc to a ferrous metal body as described in claim 2 and in which the zinc is plated on the ferrous body to a thickness within the range of about 0.0002 to 0.001 inch.

11. In the application of a protecting layer of zinc to a ferrous metal electrical conduit by electroplating in an 35-40 02/ gal. ZnSO .7H O 3-4 oz./ gal. NH Cl 1'-2 oz./gal. Al (SO 181-1 .2.5 oz./ gal. NaF pH=3.5-4.5 Free acid H 80 then bringing the bare surface of the zinc to a darker and substantially uniform color by reacting the surface metal with a reactant from the group consisting of an acidified solution of one-half percent nitric acid with 2-5 g./l. of cupric salt; an acidic solution containing zinc and iron phosphates; and free phosphoric acid, then heating the conduit to a temperature between about 800 to 950 F. by radiant heating for a period between about forty seconds and two minutes, then holding the conduit at a temperature within the range of about 800 to 950 F. for a period of about two to six minutes while subject to contact with currents of heated gas which maintain the temperature of the conduitsby contact therewith.

12. The method of applying a uniform protective layer of another metal to the surface of a ferrous metal body, which method comprises electroplating a layer of the other metal on the surface to be protected and with the ferrous metal body in an acid electrolyte, treating the electroplated surface to a state of substantially uniform reflectivity, heating the metal body and its protective layer at a rapid rate in a furnace at a temperature substantially higher than the melting point of the other metal, transferring the ferrous metal body from the highly heated space to another space of lower temperature but at a temperature higher than that of the melting point of the coating material, and holding the ferrous metal body in the other space long enough to alloy the other metal and the ferrous metal for a substantial part of the thickness of the protective layer, and in which the protecting layer that is applied to the ferrous body is 'zinc and the temperature of said other space is maintained between-800 and 950 F., and in which the Zinc is deposited with a fine-grained and small-crystalline coating by plating the metal body in an aqueous electrolyte containing 3 5-40 0Z./ gal. ZnSO .7H O 34 oz./ gal. NH Cl 1-2 oz./gal. A1 (SO .18H O .2.5 oz./ gal. NaF

pH=3.54.5 Free acid H References Cited by the Examiner UNITED STATES PATENTS 2,711,993 6/1955 Lyon 204203 2,739,107 3/1956 Ricks 20437 2,773,817 12/1956 Russell et al. 20437 2,876,176 3/1959 Pearson et al. 20437 2,921,008 1/1960 Hauck et al. 204203 3,062,725 11/1962 Frankenthal 20437 3,063,918 11/1962 Kennedy 20437 FOREIGN PATENT-S 216,056 10/1957 Australia.

JOHN MACK, Primary Examiner.

MURRAY TILLMAN, Examiner.

G. KAPLAN, L. G. WISE, W. VAN SISE,

Assistant Examiners. 

1. THE METHOD OF APPLYING A UNIFORM PROTECTIVE LAYER OF ZINC TO THE SURFACE OF A FERROUS METAL BODY, WHICH METHOD COMPRISES ELECTROPLATING A LAYER OF ZINC ON THE SURFACE TO BE PROTECTED AND WITH THE FERROUS METAL BODY IN AN ACID ELECTROLYTE, UNIFORMLY DARKENING THE SURFACE OF SAID ZINC BY CHEMICALLY TRATING IT TO BRING THE BARE SURFACE OF SAID ZINC TO A STATE OF SUBSTANTIALLY UNIFORM REFLECTIVITY, HEATING THE METAL BODY AND ITS PROTECTIVE LAYER BY RADIANT HEAT AND AT A RAPID RATE IN AN ENCLOSING SPACE AT A TEMPERATURE SUBSTANTIALLY HIGHER THAN THE MELTING POINT OF SAID ZINC, TRANSFERRING THE FERROUS METAL BODY FROM THE HIGHLY HEATED SPACE TO ANOTHER SPACE OF LOWER TEMPERATURE BUT AT AN ALLOYING TEMPERATURE OF THE FERROUS METAL AND THE ZINC, AND HOLDING THE FERROUS METAL BODY IN THE OTHER SPACE LONG ENOUGH TO ALLOY SAID ZINC AND THE FERROUS METAL FOR A SUBSTANTIAL PART OF THE THICKNESS OF THE PROTECTIVE LAYER. 